1. Field of the Invention
The invention relates to door openers and the like and refers more specifically to a torque reversing motor control structure and method for use with standard split phase or capacitive start motors wherein switching of a contactor used with the motor for reversing the motor is accomplished through a transformer, and the primary circuit of the transformer includes a centrifugal motor start switch in series therewith whereby the motor contactor cannot be reversed without the motor start switch being closed to insure positive reverse of the motor.
2. Description of the Prior Art
Continuous duty motors such as a standard split phase or a capacitive start motor have often been used for opening and closing overhead doors. Such doors present a hard starting load and once in operation requires only about one-third of the starting torque to maintain movement. Continuous duty motors have high starting torque, are constructed for continuous operation, and are available in large quantities.
The problem with standard continuous duty motors as presently used to open and close doors is that during their operation it is possible to experience limit switch overrun. Limit switch overrun with such motors used as door operators may be caused by the centrifugal start switch utilized with such motors not being engaged to provide power to the start winding of such motors when a motor contactor is switched in attempting to reverse direction of motor operation.
In this regard, the centrifugal switch is governed by the speed of the motor and does not usually engage the motor start winding until the motor speed has been reduced to approximately one-quarter of its normal operating speed.
When such continuous duty motors are commanded to stop, momentum causes the door and motor to continue to run so that the centrifugal switch is not closed when the motor contactor is reversed. Accordingly, the start winding is not energized so that the motor does not reverse but will continue to run in the same direction even though the motor contactor is now energized through a circuit controlled by a limit switch operable only with the door travelling in the reverse direction. The door continues to run in the original direction and will not stop due to opening of the limit switch in the direction of actual movement of the door. The door will jam and/or be damaged or cause damage.
Presently, the problem of limit switch overrun in such door operator installations has been solved by adding special switches or relays in the motor control circuit. These switches or relays are wired to delay motor reversing until the centrifugal switch is closed, thereby assuring reversing of the motor.